Folks, this is the situation:
Yes, you can synthesize test tones and even interesting effects, BUT you must pass through 'live music', or at least the next best thing, in order to evaluate what a fairly good quality op amp does to the music. Let us say that you have a 'perfect' CD source. Well, how do you play it back? It must be badwidth limited, simply because it is a CD and what about the op amps in the D-A stage of the CD player? They would add their own phase modulation, and potentially obscure any listening differences.

MikeB
Yes, this is correct although the open-loop vs closed-loop have been discussed somewhat at the earlier pages of this thread... As the frequency rises the "available bandwidth" for feedback decreases -> more distortion. On a similar note you can see that PSRR (of opamps for example) is dropping with frequency too (with PSSR- usually being the most sensitive due to the most common design before the output stage).

I can provide you with the mathematical background behind basic nonlinear distortion if you want to. If you know sine and cosine functions it really is very simple and very instructive as you clearly can see why THD & IMD are created.
While a 1 kHz THD measurement would probably not be very useful here (and IMO pretty useless for audio altogether unless applied to a speaker driver) – try one at 10 or 15 kHz (if your system is not bandwidth-limited to 20 kHz!).
Also, the standard CCIF (IMD test at 14 & 15 kHz) would probably yield pretty ugly numbers here!

As a next step you could cascade two nonlinear stages (say one with even order distortion and one with odd order, or just two similar ones). Now you will probably get really funky results! The math behind this pretty much just grows exponentially with the number of stages so playing around graphically (or playing the audio stream) is much more instructive!

Remember that real-world speakers have terrible distortion specs – especially the ”audiophile” crap where 5-10% isn’t uncommon. There is a reason most manufacturers don’t list the distortion specs but rather like to talk about their magic speaker wires or special green-painted resistors in their fancy pamflets.

The only speaker manufacturers I know that DO publish distortion plots are JBL, Beyma and Fane for their series of pro sound drivers. JBL:s generally have very low distortion, usually < 1% even at high SPL:s as a result of excellent and sound engineering. These are the drivers you will find if you come home to my place!

__________________"Knowing what to do but not why is no use in a changing world" - The Art of Sound Reproduction

Take all your overgrown infants away somewhere
and build them a home
a little place on their own
the Chef's Memorial Home for incurable gurus
and kings

they can appear to themselves every day
on an open loop amp
to make sure they’re still inept
it's the only topology they'll accept
"ladies and gentlemen, please welcome curl and otala
mr sleewrate and nonlinear friend leinonen
mr evil phase shift and crap
the ghost of a black gate cap
and the memories of 741:s
and now adding colour a group of anonymous digital
-hating meat packing punters"
did they expect us to treat them with any respect?

they can polish their resistors and sharpen their
ears, and abuse themselves playing games through the years
pim pim, tim tim, opamps are bad!

safe in the permanent non double-blind bubble of high-end
their favourite toy
they'll be good girls and boys
In the Chef's Memorial Home for obscurable
wasters of bandwidth and amps

is everyone in?
are you having a nice time?
now the global feedback loop can be applied.

To be released on vinyl shortly.

__________________"Knowing what to do but not why is no use in a changing world" - The Art of Sound Reproduction

I think you did not fully understood me... Of course feedback decreases
if openloopgain drops with higher freqs. I wanted to show that this
enables the "full" openloopdistortion at output even with frequencies
within the openloopbandwidth !

Okay, i did my program applying distortions to a wave. (i was already
familiar with the basical mathematic for simple unlinear distortions)
The results are very interesting and explain why 3rd harmonic is EVIL !

I took a wavefile ripped from cd, and applied 4 different distortions,
all with about 0.6%thd for a single sinewave.
Here the results:

for 2nd harmonic i use a simple y = x*(1-n) + x^2*n
for 2nd+3rd i use y = x*(1-n) + x^3*n
for 3rd only, too complex, but based on 2nd+3rd, 2nd cancelled out,
very identical to diffamp-behaviour.
And tanh speaks for itself...
All thd values were calculated with FFT. Of course these values vary with different music.

I think the first 2 numbers speak for themself, not surprising, but
explains some things !

About thd from speakers...
I made measurements with my diy-speakers, see attached pic !
The most spikes are from roomacoustics...
Observe 3rd harmonic from tweeter ! (Crossover at 2.5khz)

Folks I just ran a test measuring harmonic distortion in a sine wave generated by a function generator. I also was able to take a simultaneous fft measurement as well as a thd measurement. Then I added FM modulation to the sine wave. It could be easily seen on the fft, and the oscilloscope tracing instability, BUT it took a lot of FM to change the thd measurement. This means that the thd test is relatively insensitive to FM modulation. So there!

Was the modulation freq = the carrier freq? If the modulation freq is low, the modulation sidebands might be getting notched out if there's a notch filter for the fundamental in the distortion analyzer. And the case we're looking at is really the carrier modulating its own phase, right?

Andy C, I could use some help here, as I have never studied FM modulation formally. However, what I did find was that at a large range of modulation frequencies, I could get results on the FFT, but not with THD. This makes sense, since the THD notch is what can't follow the change in frequency.
MikeB, interesting plots. I would not have first thought the distortion quite so low, but I do believe your plots.

Normally the modulation input of these signal generators produces a peak frequency deviation proportional to the peak voltage deviation at the mdulation input. So as your modulation frequency goes up, assuming a constant amplitude of the modulating signal, the modulation index will go down proportional to modulation frequency.
The modulation sideband frequencies are the carrier plus and minus integer multiples of the modulation frequency. As the modulation index gets large, the higher-order sidebands become higher in amplitude. As the modulation index becomes very small, the f_carrier +/- f_mod will be the only sidebands seen.

So if you've got a very low modulation frequency, you'll potentially have a large modulation index, giving lots of sidebands. Yet these sidebands could be very close in to the carrier, and many could be notched out by the notch filter in the THD analyzer if it has such a filter. So I could see in this situation things could get pretty wobbly, and yet the THD analyzer might not register much in the way of distortion.

Originally posted by wimms LTSpice can be easily used to generate "interesting" wav files.

[...]
So its one example of generating sounds mathematically, by abusing spice. It can be more convenient than messing around with Matlab or wave editors. Besides, such signals could be used to feed simulated circuits.

LTSpice can also be used to "replay" wav files through your simulated circuit, so one could even play with simulations to estimate audible results.

Thanks very much wimms, that's awesome! Here I've been using LTSpice all this time and never noticed it had this capability.